1. Field of the Invention
The invention relates to the field of microfiltration membranes; it relates particularly to microfiltration membranes composed of synthetic polymers.
2. Background of the Prior Art
Highly asymmetric polymeric membranes prepared from phase separated (inversion) casting mixes have been described in patents by Wrasidlo U.S. Pat. Nos. 4,629,563 and 4,774,039, and Zepf, U.S. Pat. Nos. 5,188,734 and 5,171,445, the disclosures of which are hereby incorporated by reference. Wrasidlo discloses highly asymmetric, integrally skinned membranes, having high flow rates and excellent retention properties, prepared from a metastable two-phase liquid dispersion of polymer in solvent/nonsolvent systems. Zepf discloses improved Wrasidlo-type polymer membranes having a substantially greater number of skin pores of more consistent size, and greatly increased flow rates, with reduced flow covariance for any given pore diameter. The improved Zepf membranes are achieved by modifications to the Wrasidlo process, comprising reduced casting and quenching temperatures, and reduced environmental exposure between casting and quenching. Zepf further teaches that reduced casting and quenching temperatures minimize the sensitivity of the membrane formation process to small changes in formulation and process parameters.
A phase inversion polymeric membrane is conventionally made by casting a solution or a mix comprising a suitably high molecular weight polymer(s), a solvent(s), and a nonsolvent(s) into a thin film, tube, or hollow fiber, and precipitating the polymer by one or more of the following mechanisms: (a) evaporation of the solvent and nonsolvent; (b) exposure to a nonsolvent vapor, such as water vapor, which absorbs on the exposed surface; (c) quenching in a nonsolvent liquid, generally water; or (d) thermally quenching a hot film so that the solubility of the polymer is suddenly greatly reduced.
The nonsolvent in the casting mix is not necessarily completely inert toward the polymer, and in fact it usually is not and is often referred to as swelling agent. In the Wrasidlo-type formulations, as discussed later, selection of both the type and the concentration of the nonsolvent is crucial in that it is the primary factor in determining whether or not the mix will exist in a phase separated condition.
In general, the nonsolvent is the primary pore forming agent, and its concentration in the mix greatly influences the pore size and pore size distribution in the final membrane. The polymer concentration also influences pore size, but not as significantly as does the nonsolvent. It does, however, affect the strength and porosity (void volume) . In addition to the major components in the casting solution (mix), there can be minor ingredients, for example, surfactants or release agents.
Polysulfone is especially amenable to formation of highly asymmetric membranes, particularly in the two-phase Wrasidlo formulations. These are not homogeneous solutions but consist of two separate phases, one a solvent-rich clear solution of lower molecular weight polymer at low concentrations (e.g., 7%) and the other a polymer-rich turbid (colloidal) solution of higher molecular weight polymer at high concentrations (e.g., 17%). The two phases contain the same three ingredients, that is, polymer, solvent, and nonsolvent but in radically different concentrations and molecular weight distributions. Most importantly, the two phases are insoluble in one another and, if allowed to stand, will separate. The mix must be maintained as a dispersion, with constant agitation up until the time that it is cast as a film.
It is the nonsolvent and its concentration in the casting mix that produces phase separation, and not every nonsolvent will do this. The ones that do probably have a role similar to that of a surfactant, perhaps creating a critical micelle concentration by aligning some of the larger polymer molecules into aggregates, or colloids, which are then dispersed in the remaining non-colloidal solution. The two phases will separate from one another if allowed to stand, but each individual phase by itself is quite stable. If the temperature of the mix is changed, phase transfer occurs. Heating generates more of the clear phase; cooling does the reverse. Concentration changes have the same effect, but there is a critical concentration range, or window, in which the phase separated system can exist, as discussed by Wrasidlo. Wrasidlo defines this region of instability on a phase diagram of thus dispersed polymer/solvent/nonsolvent at constant temperature, lying between spinodal and binodal curves, wherein the polymer is not completely miscible with solvent.
Because of the great hydrophobicity of the polymer and because of the thermodynamically unstable condition of the casting mix, wherein there pre-exist two phases, one solvent-rich and the other polymer-rich (a condition that other systems must pass through when undergoing phase inversion), the unstable Wrasidlo mixes precipitate very rapidly when quenched, form a tight skin at the interface, and consequently develop into highly asymmetric membranes. Asymmetric here means a progressive change in pore size across the cross-section between skin (the fine pored side of the membrane that constitutes the air-solution interface or the quench-solution interface during casting) and sub-structure. This stands in contrast to reverse osmosis and most ultrafiltration membranes which have abrupt discontinuities between skin and substructure and are also referred to in the art as asymmetric.
Polymeric membranes can also be cast from homogeneous solutions of polymer. The composition of these formulations lie outside of the spinodal/binodal region of the phase diagram of Wrasidlo. Membranes cast from homogeneous solutions may also be asymmetric, although not usually to the same high degree of asymmetry as those cast from phase separated formulations.
Increasing the surface pore size of membranes has been described. See UK Patent No. 2,199,786 to Fuji (herein "Fuji") . The prior art teaches exposing the cast polymer solution to humid air in order to cause a phase inversion at a point below the surface of the membrane. See Fuji. The membranes produced in accordance with the Fuji process have a characteristic structure of relatively wide pores on the surface (i.e., 0.05-1.2 .mu.m), followed by progressively constricting pore sizes to the phase inversion point below the surface, followed by an opening of the pores until an isotropic structure is achieved progressing to the cast surface (i.e., 1-10 .mu.m). Accordingly, the Fuji membranes can be thought of as having reverse asymmetry from the skin surface to the point of inversion and asymmetry progressing into an isotropic structure. The patent expressly teaches that minimal asymmetry should be used in order to prolong the life of the membranes. See Page 4, Lines 7-29. Further, it appears as though the Fuji membranes are generally prepared with formulations having relatively high viscosities. For example, the polymer concentrations are usually quite high and in many cases, the membranes are prepared using polymers as non-solvents. See Example 2, page 12; Example 3, page 15.
Synthetic polymer membranes are useful as highly retentive, highly permeable filters in many testing applications in the food and beverage industry, and in medical laboratories. Many of these operations would be more cost effective and more commercially attractive if the filtration range of the membranes could be extended over the existing Wrasidlo and Zepf-type membranes.